I like the idea of having tags in the core, so long as it doesn't replace categories.
But I also like the way that the Cloudy plugin does this - it just goes through your posts and AUTOMATICALLY adds a tag cloud of your most used words and phrases. This is a hell of a lot less work. Granted, it doesn't have the power of tags and it doesn't catch related terms, but it's a hell of a lot less work and much easier to teach customers. Writers will baulk less at the amount of work required to just write a post.